It is well known that during processing of a variety of products such as wood, tobacco and food the moisture content in the product plays an important role before the product enters the final stage in the production.
When the moisture content is measured by using microwave radiation, the microwaves interact with the water molecules in the substance being measured. Due to the dipole character of the water molecule, the microwave fields interact with the molecules resulting in a rotational and translational motion of the molecules causing heat absorption of the incident energy. By measuring the attenuation (loss of energy) of the microwaves along with the phase shift (loss of velocity) of the microwaves the moisture content of the material can be accurately determined. This is typically done by transforming the output signal to an electrical signal. The attenuation and phase shift within a material can be used to calculate the dielectric properties of that material. The dielectric properties of a material are usually expressed by the relative complex permittivity, ε=ε′+jε″, where ε′ is the dielectric constant that represents the ability of a material to store electric energy, and ε″ is the loss factor representing the loss of electric field energy in the material. By knowing both ε′ and ε″ one can calculate water content and density of the material according to published formulae.
Furthermore, other physical parameters such as fat, protein and salt can be determined, for example by using more than one frequency and isolating the absorption effects due to water and absorption due to the presence of salt ions, which are governed by the difference in the frequency dependence of the two loss mechanisms. By doing repeated analysis with the device and by comparing results obtained by using conventional methods, calibration can be achieved.
Hitherto known methods using microwaves generally only measure moisture content but not other physical parameters such as fat, protein and salt. For example, no device has been made to measure fat content using non-contact microwave techniques. A hand held device has been made that estimates fat content using microwaves. However, the device needs to be in firm contact with the substance to be measured and actually measures the moisture content. The device is precalibrated and calculates the fat content from the measured moisture content.
Other systems and methods have been developed for measuring the moisture content of material. One is to use a hand held instrument, similar to the one mentioned above, a so called “stripline” sensor. The instrument is placed on the material in such a way that the stripline is in close contact with the material. Microwaves are then generated and fed along the stripline, and the attenuation is measured in the stripline. The attenuation or loss is then converted to water content. This method is a surface measurement. To obtain overall moisture content in bulk material, it is necessary to measure at several places, and turn the object around. The average value is then used as an indicator.
In another device the material is placed between the transmitting and receiving antennas of a microwave transmission system and by comparing the output signal from the material with the source signal the material properties can be deduced.
In U.S. Pat. No. 4,578,998, there is disclosed a microwave system using different polarization of signals. Two radiators are used to measure across a sheet material by utilizing two different polarizations so that signal interchange between them is avoided. The polarization is in other words used to distinguish between radiators.
The problem with the disclosure of U.S. Pat. No. 4,578,998 is that the measured signal comprises both the attenuation through the material and also reflections from microwaves bouncing off surrounding material, which is not being examined. This will introduce errors into the result.
The disadvantage of using the hand held contacting instrument is that it is a surface measurement of a bulk material. Therefore it is time consuming to obtain a measurement for the material as a whole due to the fact that one has to measure on various spots around the material and also due to the inconvenience of having a human operating the instrument by placing the sensor in contact with the material while measuring it. This can cause errors since people will never operate the instrument in exactly the same way.